Question

$\cos 4\theta -\cos 6\theta$ equals to

• A. $2\sin 5\theta \cos \theta$
• B. $-2\sin 5\theta \sin \theta$
• C. $2\sin \theta \cos 5\theta$
• D. $2\sin 5\theta \sin \theta$

Answer 1

The correct option is D $2\sin 5\theta \sin \theta$

$\cos 4\theta -\cos 6\theta =2\sin \left(\frac{4\theta +6\theta }{2}\right)\sin \left(\frac{6\theta -4\theta }{2}\right)=2\sin 5\theta \sin \theta$